FIG. 1 illustrates a section of a conventional electromagnetic drive unit 1, including a yoke 10 with a coil 11 placed around the middle leg 12, and an armature 15. When a current, preferably controlled by a control unit 99, is led through the coil 11, the yoke 10 is magnetized and thus pulls the armature 15 towards itself until the outer pole legs 16, 17 of the armature 15 clack onto the outer pole legs 13, 14 of the yoke 10.
It is generally required that electromagnetic drive units of this kind need to last millions of operation cycles where the electromagnetic drive unit is activated and then deactivated, especially when used in electromechanical switching devices, in particular in contactors. Referring back to FIG. 1, which shows also a simplified electromechanical switching device 50, where electromagnetic drive units are used to drive movable contact pieces 21, preferably placed on a movable contact bridge 20, to and from stationary contact pieces 6 in order to close or open a current path, such as between terminals 5a and 5b. The armature 15 preferably moves the contact bridge 20 via a bar 7.
In order to avoid arcing between the movable contact pieces and the stationary contact pieces, the contacts of the electromagnetic switching device need to be moved relatively fast. The pulling force of the armature 15 has to overcome the high forces of the resilient damping members 27, such as contact springs. Consequently, the resulting clacking of the armature 15 to the yoke 10 causes material fatigue especially around the points of contact, denoted in FIG. 1 with reference numeral 18. To compensate the clacking, a damping system, preferably with a resilient damping member 27, is commonly used.
To make the armature lighter, such as in the manner proposed in DE 10 331 339 A1, provides some advantage because the impact caused by the clacking can so be reduced. In this kind of implementation, especially if combined with a solution proposed in EP 1 101 233, the armature can at least partly be made of powder magnetic material, and further be hardened by using suitable polymers, like epoxy resin. A further advantage of this kind of solution is a better versatility for the shape of the armature and yoke, in contrast to prior solutions in which the armature and yoke were made of stapled metal sheets allowing simple shapes only.
A drawback of a solution of the above kind is that the proposed material for the yoke and the armature is brittle and therefore not resistant enough against impacts, therefore severely limiting the expected life time of the electromagnetic drive unit and thus not being very suitable for use in an electromechanical switching device.
U.S. Pat. No. 6,533,240 B1 discloses an electromagnetic drive unit comprising a yoke, a coil and a movable armature, whereby the yoke and the armature have a matched shape so that, when the coil is activated, the armature is adapted to at least partially cross the yoke.